We have developed a new method for regional preservation of the myocardium during periods of aortic cross-clamp by selective perfusion of the anterior descending and the circumflex coronary arteries with two different cardioplegic solutions. Ultrasonic crystals are used to assess regional function and compliance. Our first goal is to determine the best solution for functional protection of the canine myocardium. In our pilot study, cold saline coronary perfusion was inferior to cold, high potassium (30 mEQ/L) coronary perfusion. The next step is to determine the optimal concentration of potassium. Secondly, we will investigate the optimal pressure with which the coronaries should be perfused with cardioplegic solutions. The third comparison will be between blood and crystalloid potassium cardioplegia. The fourth study will involve the addition of membrane stabilizing agents, corticosteroids or procaine. The second objective of this proposal is to determine the biochemical and histologic correlates of good myocardial preservation, studying regional blood flow, electron microscopy, myocardial ATP levels, creatine phosphate levels, myocardial sodium/potassium ratios, and myocardial tissue water after periods of myocardial preservation. If any of these variables correlate highly with functional preservation, then those variables could be used with human myocardial biopsies to determine the optimal solution for clinical use. The third objective of this study is to determine the hemodynamic characteristics of myocardium which has been preserved by cardioplegic solutions, since it is likely that this myocardium will differ from normal myocardium, an important postoperative clinical consideration. The unique advantages of this experimental model are: (1) sufficient sensitivity to allow functional comparisons between two good cardioplegic solutions, and (2) the use of each animal as its own control, so that a relatively small number of experiments will be needed for each comparison.